I’m joining this thread late and to be honest, have not read all the correction details. I offer another approach with get_in that may or may not be of interest. I like these sorts of data manipulation puzzles though 
iex(11)> data1
[
%{id: "111", name: "name1"},
%{id: "222", name: "name2"},
%{id: "333", name: "name3"},
%{id: "444", name: "name4"}
]
iex(12)> data2
[
%{mapData: %{"111": "data1", "222": "data2_test", "333": "data3_test"}},
%{mapData: %{"111": "data11_test", "222": "data22_test"}},
%{mapData: %{"111": "data111_test"}}
]
iex(13)> Enum.flat_map(data1, fn %{id: id} = d1 ->
...(13)> id2 = String.to_atom(id)
...(13)> addrs = data2 |> get_in([Access.all(), :mapData, id2]) |> Enum.reject(&is_nil/1)
...(13)> addrs = if addrs == [], do: [""], else: addrs
...(13)> Enum.map(addrs, &Map.put(d1, :address, &1))
...(13)> end)
[
%{address: "data1", id: "111", name: "name1"},
%{address: "data11_test", id: "111", name: "name1"},
%{address: "data111_test", id: "111", name: "name1"},
%{address: "data2_test", id: "222", name: "name2"},
%{address: "data22_test", id: "222", name: "name2"},
%{address: "data3_test", id: "333", name: "name3"},
%{address: "", id: "444", name: "name4"}
]
